March 2016

PTFE Makes an Excellent Material for FDA Gaskets

Buying gaskets for dairy, brewery and food processing equipment is difficult. The choice is limited to FDA gaskets or FDA approved gasket material because it’s essential to avoid tainting or contaminating the product. Cleaning and sterilization routines are a way of life, so gasket material must stand up to high temperatures and caustic cleaning agents. And last, clamping forces are low in equipment like kettles, mixers and sanitary pipe fittings, so the material must be soft.

Polytetrafluoroethylene, (PTFE) sometimes referred to as Teflon®, meets all these requirements. That’s why it’s used for gasketing throughout the food industry. PTFE is listed by the FDA under 21 CFR 177.1550, although this approval really only covers virgin PTFE material. PTFE with markings, adhesive backing or filler can be used in food applications, providing the inks, adhesives and fillers also meet FDA standards. If purchasing gaskets like these for a food industry gasket application, have the material supplier confirm they qualify as FDA gaskets.

Some gasket materials will absorb traces of food preservatives, but PTFE is almost completely non-reactive. It doesn’t pick anything up and neither does it pass anything over so there’s no risk of food being contaminated with traces of elastomers or cleaning agents. The Shore D hardness of PTFE is around 50, so it’s soft and easily compressible. That’s important when pipes and vessels are secured with clamps rather than bolted flanges. PTFE also has the advantage of retaining its properties at temperatures as high as 400°F (204°C). That’s why it survives steam cleaning.

PTFE is not perfect though. It has a tendency to creep, which could lead to reduced clamping loads. This is because, unlike elastomeric gasket material, PTFE doesn’t cross-link, so bolted joints with PTFE gaskets may need occasional re-torquing.

In food processing it’s essential to use FDA gaskets. These will typically be PTFE, although some other materials are FDA approved too. If a gasket is going to contact food products it’s always best to discuss the application with the material supplier and make sure the correct material is chosen.

Does Your Application Need a Graphite Seal or Gasket?

High temperatures challenge many gasket materials. Nitrile gaskets will go up to about 95oC, silicone gaskets to 200oC and PTFE to 260oC, but what if you need to go higher? One option is to go with a metal gasket. A better one is to ask about graphite. Graphite seals and gaskets retain their properties at temperatures as high as 450oC, and have some other very useful sealing characteristics.

Properties of graphite

Graphite is a form of carbon where the atoms are arranged in layers or sheets. That lets them slide over one another easily, which translates to a slippery feel when rubbed between finger and thumb. (It’s also what lets a “lead” pencil write – graphite actually rubs off onto the paper.)

This slipperiness or low coefficient of friction is useful in gasketing or sealing. As mating surfaces are brought together graphite allows some slip. That accommodates any rotation or sideways movement as the clamping load goes on without damaging the sealing material. Used as a dynamic or shaft seal the low friction properties of graphite minimize energy losses and heat build-up while maintaining an effective barrier.

Graphite is also a very soft material, (which might seem odd when you consider it’s a cousin of diamond,) but that lets it flow into surface irregularities, which is what provides the sealing function.

Like PTFE, graphite is quite inert. It resists attack from most corrosive chemicals, even at high temperatures, and likewise doesn’t contaminate them.

Graphite seals and gaskets

It’s possible to buy graphite for use as a gasket or seal. It’s also used as a coating for some metal gaskets, (Kammprofile gaskets are an example,) where it provides excellent sealing performance along with temperature and chemical resistance. If you’re looking for gasket material for a high temperature application, ask Hennig Gasket if graphite seals might be right for you.

Failure Mechanism at High Temperatures

Previous blog posts emphasized the importance of assessing the peak temperatures anticipated in a gasketed joint. Left unsaid has been why this matters. It’s probably obvious that polyurethane, nitrile and silicone gaskets all have a temperature at which they melt. More important, all will likely fail under prolonged exposure to temperatures near their melting point, due to a phenomenon called “creep.”.

Viscoelastic materials

Most gasket materials are viscoelastic. The “viscous” part means they have a propensity to flow slowly, like a thick gel and “elastic” refers to their ability to stretch and return to their original dimensions. However, elasticity has its limits. If the material is stretched too far it can’t return to its original size or shape, resulting in permanent “plastic deformation.”

Place a viscoelastic gasket material like polyurethane under load and it becomes thinner while simultaneously spreading outwards. This is “creep.” Releasing the load lets the material recover, but only to the extent that it has not been deformed plastically.

Creep relaxation

In a gasketed joint the material is compressed, either by the stretch of the flange bolts as they are tightened or by other retaining clamps. When first placed under load it starts to creep, but as the gasket thins the load lessens until the creep stops. This is termed creep relaxation. With good design this happens before the gasket reaches a point where the joint starts to leak.

Higher temperatures

Creep is related to temperature. When a polymer like polyurethane or styrene butadiene rubber gets warmer the molecular chains slide more readily. As a result it takes less force to produce a given movement. As the temperature approaches the melting point of the material, the force needed to produce a given movement falls quickly. To take one example, this means that at temperatures over 200 F (93 C) a nitrile gasket starts shows considerable creep.

Consider the material properties

Always select gasket material with the knowledge of the maximum temperatures expected. The more safety margin can be incorporated the less creep will be experienced, leading to a longer lasting gasket.